This invention relates to shadow mask type color picture tubes, and particularly to variations in the aperture patterns of shadow masks within such tubes having corrugated apertured masks.
In a recently suggested shadow mask type color picture tube disclosed in U.S. Pat. No. 4,072,876, issued to A. M. Morrell on Feb. 7, 1978, a mask corrugated in the horizontal direction is incorporated in combination with a flat or substantially flat faceplate. The apertures of the corrugated mask are slit-shaped and are aligned in vertical columns. Within the columns, the apertures are separated by bridges in the mask either called tie-bars or webs.
The vertical spacing from the center of one web to the center of the next web within a column is the web repeat distance. To date, all commercial conventional tubes, that is tubes having a spherically curved faceplate and mask, have maintained a constant web repeat distance throughout the mask. Two patents, U.S. Pat. No. 3,766,419 issued to R. L. Barbin on Oct. 16, 1973 and U.S. Pat. No. 4,127,791 issued to J. G. van Lent on Nov. 28, 1979 have suggested that the web repeat distance in a conventional type tube could be varied in some manner over a mask. In the Barbin patent, the web repeat distance is permitted to vary randomly to prevent the formation of a moire pattern during tube operation. Moire is an undesirable woodgrain pattern appearing on the screen of a shadow mask color picture tube that is related to the interrelation of scan line spacing and web location. In the van Lent patent, the web repeat distance is varied according to a specific equation to eliminate the formation of irregularly shaped phosphor lines.
It has been found that if a tube, as suggested by the Morrell patent, is constructed with a mask having a uniform web repeat distance throughout, a moire pattern will form during tube operation. If the teaching of the Barbin patent were applied to such tube, it has been found that although moire is eliminated, the random pattern is visually objectionable. The teaching of the van Lent patent is inapplicable since it solves a problem associated with spherical masks which does not occur in the corrugated mask suggested by Morrell and the disclosed equations would not prevent moire in a corrugated mask tube.
Therefore, a need exists for a slit pattern on a corrugated mask which will eliminate or at least minimize the formation of a moire pattern in an operating tube.